Keep in mind that during the next few years the potential size of the
character set and hence of a character class is likely to increase from
about 256 to about 65536. Converting the Plan 9 regular expression stuff
to Unicode wasn't trivial. (But I didn't do the work and don't know what
techniques were used.)

Another note: the regular expression algorithm described by Thompson was
patented; the patent must have issued around 1973. AT&T never prosecuted
it, and it has probably expired; I mention it as a curiosity, and as an
illustration that software patents are not especially new.